This invention relates to organometallic ruthenium complexes which are useful for forming a ruthenium-containing thin film by the chemical vapor deposition method (hereinafter referred to as the CVD method), the coating heat decomposition method and so on, a process for producing the same and a process for producing a ruthenium-containing thin film to be used in electronics devices such as semiconductor memories.
With the progress of fine processing in memory cells in association with the recent tendency toward highly integrated semiconductor memory devices, attempts have been made to use ferroelectric thin films such as (Ba,Sr)TiO3 as insulating films in capacitors. In capacitors provided with ferroelectric thin films, use is made of noble metals such as Pt, Ru and Ir as electrodes. Among these metals, Ru is expected as the most useful electrode material, since its oxide has a conductivity and excellent fine processing properties. Thus, examinations have been made on electrodes made of an Ru thin film or an RuO2 thin film. To form such ruthenium-containing thin films in highly integrated memory devices, the CVD method is the most suitable since it is excellent in step coverage and composition controlling.
It is considered that organometallic compounds, which have low melting point and can be easily handled among metal compounds, are adequate as precursors for forming thin films by this CVD method. As organometallic compounds for the deposition of ruthenium or ruthenium oxide thin films, it has been a practice to use ruthenocene or tris(dipivaloylmethanato)ruthenium (hereinafter referred to as Ru(DPM)3) [Japanese Patent Laid-Open No. 283438/1994]] or tris(octane-2,4-dionato)ruthenium (hereinafter referred to as Ru(OD)3) [Japanese Patent Laid-Open No. 2000-212744]. Ruthenocence has a sandwich structure carrying ruthenium sandwiched between two cyclopentadienyl rings each consisting exclusively of carbon and hydrogen. Because of being highly stable in the atmosphere and having no toxicity, ruthenocene is suitable as a CVD precursor. However, it suffers from difficulties in the vaporization of the precursor and transportation onto substrates somewhat, since it is in the state of a solid at ordinary temperature and has a relatively high melting point of about 200xc2x0 C.
Accordingly, studies have been vigorously made in recent years on ruthenium compounds having lower melting point. The melting point of a ruthenium-containing organometallic compound can be lowered by converting it into a ruthenocene derivative in which at least one of the hydrogen atoms in the cyclopentadienyl rings of ruthenocene is substituted by an alkyl group such as a methyl or ethyl group. For example, Japanese Patent Laid-Open No. 35589/1999 discloses bis(alkylcyclopentadienyl)ruthenium typified by bis(ethylcyclopentadienyl)ruthenium (hereinafter referred to as Ru(EtCp)2) and bis(isopropylcyclopentadienyl)ruthenium as a ruthenocene derivative. Further, Japanese Patent Laid-Open No. 2000-281694 discloses use of alkyl-substituted ruthenocenes as CVD precursors. It is stated that each of these organometallic compounds is in the state of a liquid at ordinary temperature and has a lower melting point than ruthenocene, i.e., having characteristics required as precursors suitable for the CVD method. However, these bis(alkylcyclopentadienyl)rutheniums fundamentally have the ruthenocene structure. Since this structure has an extremely high stability, these complexes have high decomposition points. Therefore, it is essentially needed to elevate the substrate temperature to a high level in the step of film-formation, which results in a problem of worsening in the step coverage.
On the other hand, R. Gleiter et al. (Organometallics, 8, 298 (1989)) reported (cyclopentadienyl)(2,4-dimethylpentadienyl)ruthenium as an example of the synthesis of a half-sandwich complex having a cyclopentadienyl group as a ligand. However, this complex cannot be considered as suitable as a CVD precursor, since it has a melting point of 136 to 137xc2x0 C. and occurs as a solid at ordinary temperature. There has been reported no case of synthesizing a half-sandwich ruthenium complex which is in the state of a liquid at room temperature and shows excellent vaporization properties.
To synthesize a half-sandwich ruthenocene, it has been a common practice to add a pentadiene derivative, a cyclopentadiene derivative, zinc and halogenated ruthenium hydrate to an appropriate solvent at once and then reacting under appropriate reaction conditions. However, this method is not practically usable since only an extremely low yield can be achieved thereby. As the post-treatment following the reaction, it has been also a practice that the liquid reaction mixture is concentrated to give a pasty mixture and the target product is extracted from the pasty mixture with the use of an appropriate solvent followed by purification by filtration through celite or column chromatography using an alumina column to thereby give the target product. However, this method involves industrially unfavorable steps such as the extraction from the pasty mixture obtained by concentration after the completion of the reaction and the celite-filtration or the column chromatography. To make half-sandwich organometallic ruthenium compounds industrially advantageous, therefore, it has been urgently required to establish a production process whereby a target product can be obtained in a stable state at a high yield.
On the other hand, film-formation by the coating heat decomposition method has been applied to the production of elements having a relatively low integration level. Since precursors to be used in the coating heat decomposition method are dissolved in organic solvents before using to thereby control the film thickness, it is favorable that these precursors are soluble in organic solvents and decompose at low temperature. However, there have been few ruthenium compounds having the above characteristics.
There has been known no carbonylbis(diene)ruthenium complex other than carbonylbis(1,3-butadiene)ruthenium (D. Minniti and P. L. Timms, J. Organomet. Chem., 258, C12(1983)), carbonylbis(2,3-dimethyl-1,3-butadiene)ruthenium and carbonylbis(1,3-cyclohexadiene)ruthenium (D. N. Cox and R. Roulet, Helv. Chim. Acta, 67, 1365 (1984)). These complexes are produced by a process with a need for a reaction at a low temperature, i.e., reacting Ru with a diene at xe2x88x92196xc2x0 C. and then adding CO.
In the CVD method, a complex employed as a thin film precursor should be supplied as a gas. Among the complexes employed hitherto, Ru(DPM)3 is to be vaporized by sublimation because of its high melting point of 168xc2x0 C. In the vaporization by sublimation, there arises a problem that the precursor gas concentration varies depending on changes in the surface area of the solid and thus the precursor gas cannot be stably supplied. To overcome this problem, Japanese Patent Laid-Open No. 132776/1993 proposes a method wherein a complex is dissolved in an organic solvent before using. However, the precursor can be not always supplied stably by this method too, since there arise some problems that the solvent alone is vaporized or the solid is deposited due to a difference in vaporization properties between the solvent and the complex. On the other hand, Ru(OD)3 and Ru(EtCp)2 suffer from no problem in the stable supply of the precursors, since they each occurs as a liquid at room temperature and has a relatively high vapor pressure. In these complexes, however, Ru is stably bonded to the respective organic ligands. Thus, these complexes are hardly decomposed and should be treated at a high temperature for the film-formation.
The present invention aims at providing ruthenium complexes which can be subjected to the film-formation by the CVD method at a low temperature compared with the complexes as described above and can suitably supply a precursor thereof, a process for producing the same and a process for producing a ruthenium-containing thin film.
The present inventors have conducted intensive studies to overcome the above-described problems. As a result, they have found out that the decomposition temperature of a known compound having the ruthenocene structure can be lowered by substituting one of the cyclopentadienyl rings (hereinafter referred to as Cp rings) by linear pentadienyl. As the results of the subsequent studies, they have successfully developed novel ruthenium complexes, which show such a melting point as occurring as a liquid at room temperature and have favorable vaporization properties and decomposition properties, by introducing a lower alkyl group into the Cp rings. They have furthermore found out that the above-described objects can be established by carbonylbis(diene)ruthenium complexes having a low-molecular weight diene and a carbonyl group as a ligand, thereby completing the present invention.
Accordingly, the present invention provides a half-sandwich organometallic ruthenium compound characterized by being represented by the following general formula (1): 
wherein R1, R2, R3 and R4 are the same or different and each represents hydrogen, a halogen atom, a lower acyl group, a lower alkoxy group, a lower alkoxycarbonyl group or a lower alkyl group, provided that the case where R1 to R4 are all hydrogen, and the case where R1 is hydrogen, one of R2 to R4 is hydrogen, and the remainder are methyl groups are excluded.
The present invention further provides a process for producing a ruthenium-containing thin film characterized by using the above-described half-sandwich organometallic ruthenium compound as the precursor and forming a ruthenium-containing thin film on a heated substrate by the chemical vapor deposition method.
The present invention further provides a process characterized by reacting an open ruthenocene represented by the following general formula (3): 
wherein R2, R3 and R4 are the same or different and each represents hydrogen, a halogen atom, a lower acyl group, a lower alkoxy group, a lower alkoxycarbonyl group or a lower alkyl group;
in a solvent in the presence of zinc with a cyclopentadiene represented by the following general formula (4): 
wherein R1 represents hydrogen, a halogen atom, a lower acyl group, a lower alkoxy group, a lower alkoxycarbonyl group or a lower alkyl group; to thereby produce a half-sandwich organometallic ruthenium compound represented by the general formula (1).
Furthermore, the present invention provides a carbonylbis(diene)ruthenium complex characterized by being represented by the following general formula (7): 
wherein R5 to R8 represent each hydrogen, an alkyl group, or an alkyl group containing an alkoxy group, an alkoxycarbonyl group, an alkanoyl group, a hydroxyl group, a carbonyl group, a halogen atom, a carboxyl group, an amino group or a carbamoyl group, and the unsubstituted alkyl group and substituted alkyl group each has 1 to 6 carbon atoms, provided that the case where R5 to R8 are all hydrogen, and the case where R5 and R8 are hydrogen and R6 and R7 are methyl are excluded.
Furthermore, the present invention provides a process for producing the carbonylbis(diene)ruthenium complex as described above characterized in that a ruthenium chloride n-hydrate (wherein n is a number of 1 or more) is reacted with a diene in an alcohol in the presence of a zinc powder.
Furthermore, the present invention provides a process for producing a ruthenium-containing thin film characterized by using the carbonylbis(diene)ruthenium complex as described above as the precursor.